1. Field
The disclosed concept pertains generally to circuit breaker trip mechanisms and, more particularly, to such trip mechanisms including a bimetal and a magnetic armature. The disclosed concept further pertains to circuit breakers including such trip mechanisms.
2. Background Information
Electrical switching apparatus, such as circuit interrupters, include an operating mechanism and a trip mechanism, such as a thermal trip assembly and/or a magnetic trip assembly. For example, the trip mechanism is automatically releasable to effect tripping operations and manually resettable following tripping operations.
Examples of circuit breakers including trip mechanisms are disclosed in U.S. Pat. Nos. 5,805,038 and 6,838,961, which are incorporated by reference herein. Such circuit breakers, commonly referred to as “miniature circuit breakers,” have been in use for many years and their design has been refined to provide an effective, reliable circuit breaker which can be easily and economically manufactured and tested.
As is well known, circuit breakers of this type include, for example, at least one set of separable contacts disposed within a non-conductive housing. Typically, there is a fixed contact coupled to the housing and a movable contact coupled to the operating mechanism. The operating mechanism includes a movable handle that extends outside of the housing. Movement of the separable contacts is accomplished by the operating mechanism. The operating mechanism typically includes components such as the previously mentioned handle, an operating arm, upon which the movable contact is disposed, a cradle, and the trip mechanism, such as the previously mentioned thermal trip assembly and/or magnetic trip assembly. The cradle is coupled to a spring and disposed between the trip mechanism and the operating arm. The components may further include a frame to which the other components are coupled.
The circuit breaker is magnetically tripped automatically, and instantaneously, in response to overload currents above a predetermined value higher than another predetermined value for a thermal trip. Flow of overload current above the higher predetermined value through a bimetal induces magnetic flux around such bimetal. This flux is concentrated by a magnetic yoke toward an armature. An overload current above the higher predetermined value generates a magnetic force of such a strength that the armature is attracted toward the magnetic yoke resulting in the flexing of a spring permitting the armature to pivot, release the cradle and trip the circuit breaker open.
Typically, the circuit breaker includes the thermal trip assembly such as a bimetal assembly. When the circuit breaker is closed, a persistent overload current of a predetermined value causes the bimetal to become heated and deflect away from the cradle. The armature, which is supported on the bimetal by a leaf spring, is carried with the bimetal to release the cradle and trip the circuit breaker in a well known manner.
The armature includes an armature window having a latching surface that normally engages a corresponding latching surface of the cradle. During interruption, when the armature is attracted toward the magnetic yoke, the armature can engage the surface of the bimetal. The armature window can fill with molten metal from arc splatter. This can result in the armature being tack welded to the bimetal.
There is room for improvement in trip mechanisms.
There is also room for improvement in circuit breakers including a trip mechanism.